On a primary, sets of XLOG_RUNNING_XACTS records are generated on a periodic basis to allow recovery to build the initial state of transactions for a hot standby. The set of transaction IDs is created by scanning all the entries in ProcArray. However it happens that its logic never counted on the fact that two-phase transactions finishing to prepare can put ProcArray in a state where there are two entries with the same transaction ID, one for the initial transaction which gets cleared when prepare finishes, and a second, dummy, entry to track that the transaction is still running after prepare finishes. This way ensures a continuous presence of the transaction so as callers of for example TransactionIdIsInProgress() are always able to see it as alive.
So, if a XLOG_RUNNING_XACTS takes a standby snapshot while a two-phase transaction finishes to prepare, the record can finish with duplicated XIDs, which is a state expected by design. If this record gets applied on a standby to initial its recovery state, then it would simply fail, so the odds of facing this failure are very low in practice. It would be tempting to change the generation of XLOG_RUNNING_XACTS so as duplicates are removed on the source, but this requires to hold on ProcArrayLock for longer and this would impact all workloads, particularly those using heavily two-phase transactions.
XLOG_RUNNING_XACTS is also actually used only to initialize the standby state at recovery, so instead the solution is taken to discard duplicates when applying the initial snapshot.
1df21ddb19 Avoid duplicate XIDs at recovery when building initial snapshot
src/backend/storage/ipc/procarray.c | 21 +++++++++++++++++++--
1 file changed, 19 insertions(+), 2 deletions(-)